This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Enveloped viruses use specialized protein machinery to fuse the viral membrane with that of the host cell in a programmed fashion. Influenza A virus is a paradigm for understanding viral entry of host cells via receptor-mediated endocytosis. The hemagglutinin (HA) envelope glycoprotein mediates influenza virus membrane fusion. While certain facets of HA-mediated fusion have been characterized, significant gaps remain in the structural characterization of the molecular conformations that actually drive membrane fusion, in our understanding of how HA switches conformations, and in our understanding of the interplay between HA conformational change and membrane deformation.